In relation to the conventional wireless units of this type, there have been reported adverse effects on a human body due to exposure of the human body to a high-intensity electromagnetic wave in recent years. The reported adverse effects include an elevation of the temperature of the human body, and stimulation of a nerve in the human body. In consideration of the adverse effects on the human body, a regulation has been established since June, 2002 to ensure that a specific absorption rate (SAR) must be 2 mW/g or less. The SAR is the amount of energy of an electromagnetic wave transmitted by a wireless unit, such as a mobile phone, during a phone call and absorbed by a specific part (especially, a head) of a human body for six minutes, and is measured as an average over a 10-gram cube of a human tissue.
The SAR is expressed as σE2/ρ, where σ is a conductivity (Siemens/meter) of a human tissue, E is a field intensity (Voltage/meter) of an electric field applied to the human body, and ρ is an intensity (kg/m3) of the human tissue. The SAR can be reduced by a reduction in the field intensity of the electric field, which varies depending on radiation power emitted from a wireless unit. When the radiation power emitted from the wireless unit is reduced, the SAR is also reduced. The reduction in the SAR, however, may result in a deterioration of the quality of communications performed by the wireless unit.
In order to reduce the SAR without a reduction in the radiation power emitted from the wireless unit, there has been proposed a technique in which a conductive plate having a predetermined shape and a circuit board are short-circuited by a conductive material (refer to Patent Document 1).
FIG. 22 is a diagram showing the construction of a conventional antenna apparatus mounted in a mobile wireless unit including: a case 901; an antenna element 902 provided outside the case 901; a feeder 903 mounted in the case 901 and feeding an antenna; a circuit board 904; a shield case 905 for covering the circuit board 904; a conductive plate 906; a short-circuit conductor 907 for short-circuiting an end of conductive plate 906 with the shield case 905.
The conductive plate 906 and the other end of the shield case 905 are in an electrically open state. The conductive plate 906 has an electrical length, which is one-fourth wavelength of a frequency. This leads to the fact that impedance between the conductive plate 906 and the shield case 905 is about zero at an end portion of the conductive plate 906, which is short circuited with the shield case 905, and is almost infinite at another end portion of the conductive plate 906, which is in the electrically open state. The construction of the conventional antenna apparatus makes it difficult to cause a high frequency current to flow from the vicinity of the feeder 903 to the conductive plate 906 and the shield case 905. This results in a reduction in the amount of radiation of an electromagnetic wave from the conductive plate 906 and the shield case 905 and in a reduction in the SAR.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-94311